1. Field of the Invention
The present invention relates generally to the field of two-phase heat transfer systems. More specifically, the present invention discloses a flowrate controller for a hybrid capillary/mechanical two-phase thermal management system.
2. Statement of the Problem
Two-phase heat transfer systems, and in particular, capillary pumped loops have substantial advantages for space applications. The basic operation of a capillary pump loop ("CPL") involves pumping a working fluid through the heat transfer system by means of the capillary forces developed in a wick material located inside the evaporator. A CPL has no moving parts and is self-controlling, in that the flow rate of working fluid through the evaporator will automatically change to match the thermal load. CPL's are ideal for managing heat loads in spacecraft where vibrations, such as those from a mechanical pump, are detrimental. In addition, CPL's offer high reliability due to the absence of moving parts. They offer automatic heat load sharing if a number of evaporators are used in parallel. Phase separation and flow distribution are automatically controlled since the flow rate through each evaporator is related directly to the rate of evaporation at the wicking surface inside. Adjacent evaporators can operate at significantly different heat input rates, but both will have only working fluid vapor at their exits. However, CPL's have disadvantages in that they cannot be readily scaled to large systems and CPL evaporators can deprime if the power loads are too high or too low.
The concept of a capillary pumped loop was developed in the mid 1960's by F. J. Stenger at the NASA Lewis Research Center (F. J. Stenger, "Experimental Feasibility Study of Water-Filled Capillary-Pumped Heat Transfer Loops," NASA TM X-1310, NASA Lewis Research Center, Cleveland, Oh., 1966). Development continued at the NASA Goddard Space Flight Center with construction of a number of CPL's beginning in the late 1970's. Several of these systems have been developed at the Goddard Space Flight Center and one has twice been flown on the space shuttle to demonstrate micro-gravity operation.
In contrast, a two-phase mechanically pumped loop ("MPL"), such as a conventional heat pump or refrigeration cycle, can be readily scaled to large, high-power applications. Aside from the additional weight, complexity and vibration provided by the pump, the other major disadvantage of an MPL is the difficulty of regulating the flowrates if a plurality of evaporators are used. A complex system of thermostats and control valves is typically employed to regulate the overall flowrate of the pump, and the flowrates through each evaporator in response to its individual thermal load.
The possibility of a hybrid capillary/mechanical thermal loop has been previously considered in which a mechanical pump boosts the pressure of the working fluid entering the evaporators, thereby effectively increasing the heat transfer capacity by permitting greater flowrates through the evaporators. However, in such a hybrid, the flowrate of the booster pump must be regulated such that the evaporators stay primed and are not flooded. Nearly all of the system pressure drops are overcome by the mechanical pump. The piping in the evaporator section need only be sized to make certain that the pressure difference across any evaporator does not exceed the pressure limit of the local capillary structure to prevent flooding or depriming of the evaporator.
The general outline of a hybrid capillary/mechanical two-phase thermal loop with a flowrate controller was previously discussed by Cullimore, et al., "Thermostatic Control of Two-Phase Spacecraft Thermal Management Systems" (AIAA/ASME 41th Joint Thermophysics and Heat Transfer Conference, June 2-4, 1986, Boston, Mass. AIAA-86-1246). Several possible embodiments were discussed in this paper. The last embodiment discussed on page 6, column 2 (see FIG. 5) mentions the possibility of using a dummy evaporator in parallel with the evaporators to monitor the liquid level in the evaporator capillaries. Capacitance sensing is noted as one possible means of monitoring the liquid level in the dummy evaporator. This dummy evaporator is described as having "a capillary structure as fine or finer than the real evaporators, and a net permeability that is less than or equal to the real evaporators." However, subsequent analysis has shown that these limitations are erroneous, and that the liquid level sensor disclosed in this paper would not function properly.
The prior art also contains several examples of two-phase heat transfer systems and capillary-pumped heat pipes, including the following:
______________________________________ Inventor U.S. Pat. No. Issue Date ______________________________________ Busey 3,866,424 Feb. 18, 1975 Eastman 4,352,392 Oct. 5, 1982 Bizzell, et al. 4,470,450 Sep. 11, 1984 Bizzell, et al. 4,492,266 Jan. 8, 1985 Niggemann 4,603,732 Aug. 5, 1986 Edelstein 4,664,177 May 12, 1987 ______________________________________
Busey discloses a heat source powered by radioactive nuclear waste in which heat pipes 20 are used as a back-up safety feature to dissipate heat in the event the primary coolant loops become inoperative.
Eastman discloses a mechanically assisted evaporator surface in which liquid is pumped to and sprayed from a nozzle onto a sintered metal layer to keep the entire surface wetted at all times so as to permit uniform thin film evaporation from the surface.
Bizzell, et al., (U.S. Pat. Nos. 4,492,266 and 4,470,4350) disclose a pump-assisted heat pipe. The evaporator 12 has a capillary structure permitting evaporation of the working fluid. The condenser 13 is connected to the evaporator 12 by a conduit. Condensed working fluid is returned from the condenser 13 to the evaporator 12 by a liquid-phase pump 11. Due to the additional pressure provided by the pump, there is no limitation on the length of the heat pipe or the conduit 14 caused by capillary pumping requirements of the system.
Niggemann discloses another example of a pumped, two-phase, heat management system for spacecraft.
Edelstein discloses yet another example of a pumped, two-phase, heat transfer system. An array of independently operating grooved capillary heat exchanger plates 20 acting as evaporators are connected in parallel. A vapor line 17 interconnects the evaporators with a condenser 12. The condensed working fluid is returned with the assistance of a pump 15 from the condenser 12 to the evaporators 20. The flowrate through each evaporator 20 is separately regulated by a valve 25 controlled by the temperature of the evaporator 20.
3. Solution to the Problem
None of the prior art references show a hybrid capillary/mechanical thermal loop having a sensor (in the form of a long coiled tube containing an insulated wire) connected in parallel with the evaporators, that permits capacitive measurement of the volume of liquid phase working fluid within the tube, which is then used to regulate the flowrate of the booster pump.